With respect to the quality of synthetic quartz glass substrates, further improvements in defect size, defect density, and surface roughness on substrates are needed to meet the demand for the size reduction of photolithography pattern. In the IC and related fields, for example, the trend toward pattern size reduction and capacity enlargement poses severe specifications with respect to defects on substrates.
Under the circumstances, it is desirable for the substrate surface to minimize defect size and reduce defect density, and even to be defect-free. Efforts are made to attain the goal, for example, by adding additives to polishing slurries to improve the dispersion state. JP-A S64-40267 discloses precision polishing of glass substrates using colloidal silica. When the polished glass surface is inspected for defects using a high-sensitivity flaw detector, microscopic raised/recessed defects are observed. This method is still unsatisfactory as the microscopic defect control.
JP-A 2009-131947 discloses a method of preventing the surface of a magnetic disk substrate from being scratched. A water-soluble polymer such as a (meth)acrylic acid/sulfonic acid copolymer is added to a polishing slurry of silica (abrasive grains) to inhibit agglomeration of silica therein. Since the long molecular chain of the polymer like (meth)acrylic acid/sulfonic acid copolymer can be gradually cleaved by the polishing heat, the polishing slurry loses stability during the continuous polishing process, which is undesired from the aspect of polishing life.
The method of JP-A 2009-131947 suffers from economic and environmental problems. Since the polishing slurry fails to maintain its quality stable during circulating repetitive service, the polishing slurry must be replaced by a fresh one. It is difficult under certain polishing conditions to estimate the lifetime of the polymer in the slurry.